Resistors, thick-film noise

A noise and nonlinearity of thin and thick film resistors are used as reliability indicators to make a prediction of resistor reliability. The resistor noise is proportional to the current density and this property makes it a valuable characteristic for detecting imperfections and abnormalities. The magnitude of the current noise is dependent upon many inherent properties of the resistor such as resistive material and technology factors as processing, size and shape. In the last decade l/f noise in resistive layers has been studied by many authors. It become evident that occurrence of l/f noise can serve as a measure of the technology standard. A problem which has been left consists in identification of the source of l/f noise. 

It is fi quently observed that excess l/f noise is related to the microscopic sample structure and to the manufactural technology. This kind of noise is particularly sensitive to the surface and interface defects (Jones 1994). There are manufactural techniques which give small dispersion of the mean characteristic values, such as resistance or currents. On the other hand these devices can exhibit large dispersion ofthe noise characteristics. So, for example, metal thin film resistors have lower l/f noise than granular film resistors made from cermet thick films or carbon resistors. 

Jevtic, M.M. et al. 1999. Thick-film resistor quality indicator based on noise index measurements. Microelectronics Journal, 30, pp. 1255-1259. 

Rocak, D. et al. 2001. Low-frequency noise of thick-film resistors as quality and reliability indicator. Microelectronics Reliability 41 (4) 531-542. 

Sedlakova, V. et al. 2003a. Current density dis-tribution, noise and non-linearity of thick film resistors, CARTS US Scotsdalle March 31, 2003. 

The electrical properties of thin-film resistors are substantially better than thick-film resistors in terms of noise, stability, and precision. 

Carbon. One of the materials first used for thick-film resistors was carbon. Early carbon-film resistors had a negative coefficient of resistance (NTC) from 0.01 to 0.05 percent per °C, whereas later resistors had an NTC from 0.005 to 0.02 percent per °C. This improvement in performance was achieved by adding boron to help prevent oxidation of the carbon. A metallic dispersion in the carbon film also improves the temperature coefficient the NTC of the carbon is balanced by the PTC of the metaUic dispersion. Carbon and boron-carbon films have the highest electrical noise level of the film-type resistors. Noise is generated by the small fluctuations in the resistance caused by imperfections, variations in the particle-to-particle contact, and hot spots. 

Resistor Noise. The random motion of current carriers causes spontaneous fluctuations in electric current, termed electrical noise. Current noise in thick-film resistors is called excess noise, as it is considered the excess noise over thermal noise. Thermal noise is predictable and is a function of resistance and temperature, whereas current noise has no definite relationship. Noise in thick-film resistors depends on the sheet resistance value, thickness of the resistor print, and geometry of the resistor. Figure 8.59 shows a typical plot of measured noise versus resistor area for a 200-kQ/sq resistor. Current noise decreases as resistor area increases. Resistor noise also depends on processing conditions. Generally, noise decreases with an increase in peak firing temperature. The typical noise value for commercial thick-film resistors is shown in Table 8.20. 

Plastic potentiomers described as robust yet economical use a conductive polymer as the resistive element of the design. The polymer used is effectively a thick screen-printed film ink similar to the cermet (ceramic/metal) compounds which are also used in variable resistors. Conductive polymers are superior to cermet in terms of their dynamic noise characteristic but have inferior moisture resistance, temperature coefficients, power dissipation and wiper current capacities. The temperature coefficient and power handling capabilities of wirewound resistors are higher. 

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